|  |
Insect Galls1
E. A. Buss2Galls occur on many different plants. These growths may be caused by insects, mites, fungi, bacteria, or nematodes, but insect galls are the most common. Gall-forming insects include adelgids, phylloxerans, psyllids, gall midges, and gall wasps. Of the more than 2,000 galling insects in the United States, over l,700 are gall midges or gall wasps.
Galls are abnormal growths of plant cells formed in response to egg-laying by adult insects or feeding by immatures. Eggs are usually laid in actively growing plant tissue. The irritated plant tissue quickly surrounds the egg or immature insect, and protects and provides food for the gall-maker until it matures. Gall-makers may live in individual or communal chambers inside the gall.
The shape of each gall depends on which insect caused it to form. They may look like tiny blisters, round balls, tubes, or folded leaves. The gall surface may be smooth, hairy, or covered with spines. They may occur anywhere on the plant (e.g., seeds, flowers, petioles, branches, stems, roots), but most galls occur on leaves. More than one type of gall may occupy a plant or plant part. Commonly infested plants are in the families Rosaceae (roses), Asteraceae (asters), Salicaceae (willows), and Fagaceae (oaks). Oaks, however, support the greatest diversity of gall-makers in North America. Gall susceptibility varies greatly between plant species grown under the same conditions.
Galling insects are usually not considered pests. Most are harmless and just look interesting. However, certain species can physically or aesthetically damage high value plants by reducing photosynthesis and seed production, discoloring foliage, causing defoliation, branch dieback, and rarely, plant death.
Gall Wasps (Hymenoptera: Cynipidae)
Gall wasps ( Figure 1 ) have unusual life cycles. Wasps of the two generations often look physically different, and may attack the same or different plant structures and make very different galls. As a result, the wasps of both generations have occasionally been falsely described as separate species. One generation is usually all female and the other has both males and females. Adult gall wasps are small (1-6 mm long). Larvae are about 1-4 mm long, white, and lack legs. Larvae pupate inside the galls, and adults chew circular holes in the galls upon emergence ( Figure 2 ). One generation may last from several months to three years.
CREDITS: J. Shorthouse, Laurentian University
Figure 1. Adult gall wasp (Diplolepis rosae) which develops in the mossy rose gall.
CREDITS: J. Castner, University of Florida
Figure 2. Gall wasp adult inside a leaf gall. Gall Midges (Diptera: Cecidomyiidae)
Gall midges are tiny, delicate flies (2-3 mm long) ( Figure 3 ). Larvae are legless (1-5 mm long), and white, yellow, or red in color. The larval stage may last from two weeks to more than two years. Feeding damage is only caused by the larvae, which suck plant juices inside galls. Some larvae pupate within their galls, while others exit the galls and pupate in the soil. Subsequent adult activity is timed to the host's biology, often coinciding with budbreak in the spring. Female gall midges may be attracted to host chemicals and physical plant characteristics (ie., leaf hairiness). Most gall midges are host-specific. The galls may be spherical ( Figure 4 ) or resemble thorns.
CREDITS: R. Gagne
Figure 3. Adult gall midge.
CREDITS: L. Buss, University of Florida
Figure 4. Galls formed by gall midges on goldenrod. Aphids (Hemiptera: Aphidae)
Aphids are pear-shaped, soft-bodied insects. Individuals of some species are covered with waxy threads, and the wings are held roof-like over the adult's body. They form galls on many plants, petioles, leaves, or twigs, depending on the species. Many aphid gall-makers use different plant genera for winter and summer hosts. Galls occur more frequently on the winter hosts, which usually are trees. Galls occur in the spring after overwintering eggs hatch. Two to several generations of aphids develop in the gall, after which winged adults leave the gall in search of summer hosts.Psyllids (Hemiptera: Psyllidae)
Psyllids, or jumping plantlice, are small (2-5 mm long) and resemble miniature cicadas. Little is known about their biology. Galls typically look like rolled leaves ( Figure 5 ), buttons, or cones, and some waxy powder may be present. Species of Pachypsylla make galls on hackberry.
CREDITS: L. Buss, University of Florida
Figure 5. Psyllid gall on a wild lime leaf. Phylloxerans (Homoptera: Phylloxeridae)
Phylloxerans are related to aphids and psyllids. However, adults hold their wings flat over the body. These insects do not produce waxy threads, but some species are covered with a waxy powder. Their life cycles are often very complex. Galls often appear as tiny blisters on leaves ( Figure 6 ).
CREDITS: R. Mizell, University of Florida
Figure 6. Pecan leaf phylloxera galls. Biological Control
Most gall-makers have a complex of specific natural enemies (parasitoids, predators, pathogens) that help suppress their populations. However, natural enemies need a little time to catch up with increasing gall-maker numbers. In addition to attacking gall-makers, some of the dult parasitoids and predators may also feed on pollen or nectar for extra energy.Galls are good nutrient sources, so other insects may feed on them or use the gall for shelter. Some of these insects can also kill some or all of the gall-makers. The gall invaders may include gall wasps that cannot form their own galls, gall midges, clearwing borers, longhorned beetles, metallic wood-boring beetles, weevils, ants, and others. Some of these opportunistic insects may also be pests on other plant species.
Cultural Control
Pruning and destroying infested plant parts is an effective, but labor-intensive, way to minimize gall problems. Without pruning, leaf galls will either drop off with the leaves or "jump" off by themselves. However, those on the branches, roots and stems of trees may persist for several years.Host plant selection is important, but greater work is needed to identify resistant plant varieties and species to specific gall-makers. Gall susceptibility is likely linked to a plant's genetic makeup and age. In general, galls are more likely to form on actively growing plant parts. Maintain normal fertilization, irrigation, and other approved practices to keep plants healthy.
Chemical Control
Because most gall-makers don't physically damage plants, chemical control is not recommended. But, if populations are a big problem, either target the adult gall-makers before they lay eggs (often at bud break) with a contact insecticides or the immatures inside the young galls with a systemic. Correct application timing is important for either tactic. Monitor adult activity by placing sticky traps near the galls, and dissect several galls a week to track insect development. Watch the plant's development, too. Most galling insects are finely tuned to plant phenology.Contact insecticides such as carbaryl (Sevin), chlorpyrifos (Dursban), and any pyrethroid insecticide may be effective against adults. Systemic insecticides (e.g., Orthene, Avid) may kill developing insects in leaf galls, but likely won't affect insects in stem galls. Dormant oil can control aphid galls. Treatments may be applied as high pressure sprays, soil drenches, or trunk injections of highly concentrated insecticides. Drift and nontarget exposure can be problems using sprays, however.
For more information:
Hackberry petiole gall psyllid (http://edis.ifas.ufl.edu/IN214 )
Citrus gall midge (http://edis.ifas.ufl.edu/IN162 )
Blueberry gall midge (http://edis.ifas.ufl.edu/IN458 )
Footnotes
1. This document is ENY-331, one of a series of the Department of Entomology and Nematology, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. First printed: October 1993. Revised: January 2003. Please visit the EDIS Website at http://edis.ifas.ufl.edu.2. E. A. Buss, assistant professor, Entomology and Nematology Department, Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, 32611.
The use of trade names in this publication is solely for the purpose of providing specific information. UF/IFAS does not guarantee or warranty the products named, and references to them in this publication does not signify our approval to the exclusion of other products of suitable composition. All chemicals should be used in accordance with directions on the manufacturer's label. Use pesticides safely. Read and follow directions on the manufacturer's label.
The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. For more information on obtaining other extension publications, contact your county Cooperative Extension service.
U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Larry Arrington, Dean.
Copyright Information
This document is copyrighted by the University of Florida, Institute of Food and Agricultural Sciences (UF/IFAS) for the people of the State of Florida. UF/IFAS retains all rights under all conventions, but permits free reproduction by all agents and offices of the Cooperative Extension Service and the people of the State of Florida. Permission is granted to others to use these materials in part or in full for educational purposes, provided that full credit is given to the UF/IFAS, citing the publication, its source, and date of publication.